1. Field of the Invention
The present invention relates to a signal processing method, a signal processor circuit, and imaging apparatus, and in particular to imaging apparatus such as a digital camera and a digital video camera equipped with an image pickup device capable of outputting a high-sensitivity signal and a low-sensitivity signal, and a signal processor circuit and a signal processing method therefor.
2. Description of the Related Art
In general, imaging apparatus such as a digital camera and a digital video camera has a specific dynamic range determined by the electric charge storage capacity of a built-in image pickup device. The dynamic range of the image pickup device is narrower than the dynamic range of a silver halide negative film obtained by using a camera which employs a silver halide film.
Thus, in case a scene is to be imaged where a very bright (high-luminance) subject and a relatively dark (low-luminance) subject are present, such as photographing an exterior from an interior, the exposure time is controlled so that wither the high-luminance subject or low-luminance subject is will be exposed at an appropriate level. Under such control, setting the luminance level of the low-luminance subject as an appropriate exposure level may result in overexposure of the high-luminance region. Setting the luminance level of the high-luminance subject as an appropriate exposure level may result in underexposure of the low-luminance region. This obtains only an inappropriate image.
In order to solve these problems, imaging apparatus has been developed which acquires two types of image signals or picture signals, that is, a high-sensitivity image or picture signal and a low-sensitivity image or picture signal each having a dynamic range different from each other and optimizes to synthesize the two types of image signals, thereby obtaining a signal having a wide dynamic range. Such imaging apparatus is disclosed for example in the Japanese Patent Laid-Open No. 2001-94999 and the Japanese Patent Laid-Open No. 2001-8104.
The Japanese Patent Laid-Open No. 2001-94999 discloses imaging apparatus which appropriately regulates a standard-luminance picture signal obtained so that a subject of a standard luminance will be at an appropriate level and a high-luminance picture signal obtained so that a subject of a luminance higher than the standard will be at an appropriate level and synthesizes both signals, thereby obtaining a wide dynamic range picture signal.
The Japanese Patent Laid-Open No. 2001-8104 discloses imaging apparatus which uses imaging means capable of acquiring both a low-sensitivity signal and a high-sensitivity signal to take a low-sensitivity picture and a high-sensitivity picture and synthesizes both signals, thereby obtaining a wide dynamic range picture signal.
On the imaging apparatus described in the Japanese Patent Laid-Open No. 2001-94999, it is necessary, during a signal synthesis process, to change the weight of a signal to be adaptively synthesized in accordance with the level of imaging data. Uniform signal synthesis for every scene would result in degraded picture quality depending on the scene. Thus, it is desirable to limit the dynamic range depending on the scene based on for example the information on the contrast of the subject, and the signal synthesis for this approach will require complicated arithmetic operation. This presents a need to design a complicated signal processor circuit for performing complicated arithmetic operation, which increase the overall cost.
On the imaging apparatus described in the Japanese Patent Laid-Open No. 2001-8104, optimization of individual images to be synthesized is required in order to perform signal synthesis and obtain a wide dynamic range picture signal. However, simultaneous optimization of the individual images requires as many optimization circuits for performing processing such as correction of gray-scale levels as the number of images to be synthesized. This increases the circuit scale and complicates the circuit design, which further increases the overall cost.
In case a correction signal for gray-scale correction is generated and gray-scale correction is made in each optimization circuit then synthesis of images is performed in a synthesis circuit, the signal processor circuit undergoes a large computation load thus increasing the processing time.